Well, if we examine what Kurtz has done based on his actions alone, it is absolutely obvious that he has done enough to warrant the description "evil." Consider his actions in dominating a tribe and then leading it on raids to other tribes to get their ivory. Kurtz deliberately made the natives think that he was a god to gain power and prestige and enable him to gain more ivory. At one stage, Marlow comments:
Evidently the appetite for more ivory had got the better of the - what shall I say? - less material aspirations.
Spot the note of irony at the end of that sentence. Interestingly, this comment is made just before Marlow examines the house of Kurtz and discovers that what at first he had considered to be ornamentation were actually heads impaled on stakes and placed around the house. Marlow interprets these heads by saying:
They only showed that Mr. Kurtz lacked restraint in the gratification of his various lusts, that there was something wanting in him - some small matter which, when the pressing need arose, could not be found under his magnificent eloquence.
So, Kurtz, for all his "eloquence" and brilliance, clearly shows the evil nature of his actions in the "heart of darkness" through his methods of being so successful and the violence that he caused to gain that success.